Charge transferring means for electrostatic generators



May 5, 1959 E. E. HAND 2,885,599

CHARGE TRANSFERRING MEANS FOR ELECTROSTATIC GENERATORS Filed Feb. 8,1955 2 Sheets-Sheet l ls v I FIG. 3

COLLECTED CURRENT (/10) SCREEN CURRENT CURRENT AVAILABLE FOR COLLECTION(#0) May 5, 1959 5, HAND 2,885,599 CHARGE TRANSFERRING MEANS FORELECTROSTATIC GENER ATORS Filed Feb. 8, 1955 2 Sheets-Sheet 2 FIG. I

United States Patent CHARGE TRANSFERRING MEANS FOR ELECTROSTATICGENERATORS Eugene E. Hand, Jamaica Plain, Mass., assignor to HighVoltage Engineering Corporation, Cambridge, Mass, a corporation ofMassachusetts Application February 8, 1955, Serial No. 486,794

Claims. (Cl. 317-2) This invention relates to apparatus for collectingelectric charge from the movable charge carrier of a highvoltageelectrostatic generator, and in particular to apparatus for collectingelectric charge, by a corona mechanism, from the traveling endless beltof an electrostatic belt-type generator. In accordance with myinvention, such apparatus includes two components, one of whichinitiates the necessary corona discharge and the other of which collectsthe major portion of the electric charge collected from the belt.

When the movable charge carrier carries a net negative charge, thecharge-collecting process is believed to involve predominantly atransfer of negative charge from the carrier to the charge-collector viathe corona discharge. When the movable charge carrier carries a netpositive charge, the charge-collecting process is believed to involvepredominantly a transfer of negative charge from the charge-collector tothe carrier via the corona discharge resulting in a neutralization ofthe net positive charge on the carrier and in the production of a netpositive charge at the charge-collector. In the following specificationand claims, the term charge-collection and related terms include boththe actual transfer of the net charge from the carrier to thecharge-collector and the apparent transfer resulting from neutralizationof the net charge on the carrier with resultant production of net chargeat the charge-collector.

In electrostatic generators of the type shown in the United Statespatent to Robert J. Van de Graafi, No. 1,991,236, dated February 12,1935, electric charge is deposited by a corona mechanism on a rapidlymoving insulating belt and carried physically to the high-voltageterminal. There it is removed by a charge collector which also employsthe corona mechanism. Similarly, the descending run of the belt may havecharge sprayed on it within the terminal which is physically carrieddown to ground potential, where it is removed in like fashion.

The method which is generally employed for spraying and removingelectric charges from such belt conveyors makes use of a row of metalliccorona points which are directed at the insulating belt and which rowextends across the width of the belt. The spraying of charge on the beltis accomplished by causing a potential difference to exist between thesepoints and the pulley over which the belt passes. That is, there iscaused an electric gradient in the region of the corona pointssufiicient to produce ionization of the gas, and to cause the movementof ions between the corona or transferring points and the surface of thecharge carrier. The high gradient in the region of the points due totheir geometry causes ionization of the gas and the movement of ions toand away from the corona points, depending upon the polarities involved.If the points are negative, for example, negative ions are directed awayfrom the points toward the pulley of the insulating belt, but areintercepted by the said intervening insulating belt and carried away.The spraying on of an electric charge is always accomplished in thevicinity of 2 the said pulley or other metallic electrode adjacent tothe belt.

For the removal of an electric charge, a similar arrangement of metalliccorona points extending across the width of the insulating belt isemployed. In this situation, however, the removing of a charge isaccomplished in a region which is remote from the belt pulley or othermetallic electrode, since it is desired that the capacitance between thecharged belt surface and the collecting electrode system be small. Whenthis is the case, the charge density on the belt results in a highdifference of potential between this charged surface and the row ofmetallic corona points, which in turn produces a high electric gradientin the region of the said points and ionization of the surrounding gas,as stated. The availability of ions in this local electric field resultsin the flow of an electric charge toward the belt, which flow tends toneutralize the charge on the belt, and a corresponding fiow of ions ofthe opposite sign towards the said corona points. Thus the charge on theinsulating belt may be neutralized and the points may collect a currentequal to the rate at which charge is brought.

In general, if a metal point be directed at a plane and be maintained ata constant voltage with respect to the plane, a current will flowbetween the point and the plane because of the ionization of the gas inthe vicinity of the point. This current will become measurable at adefinite voltage difference called the corona onset voltage, whichdepends on various factors. In particular, the corona onset voltagedecreases with decreased pointto-plane spacing. As the voltage isincreased beyond this onset value, the current increases rapidly untilultimately a voltage value is reached at which a spark will pass. In thenormal spraying and charge-removing process, the corona points areoperated in this intermediate region, in which a steady unidirectionalflow of current can be obtained from the said point.

As hereinbefore stated, the spraying of charge onto the belt isaccomplished by applying a potential difference between the spray-oncorona points and the pulley over which the belt passes. A relativelylarge potential difference is readily available for this purpose, sothat considerable latitude is permissible in the construction of thespray-on corona points and in the spacing between the spray-on pointsand the belt. In two-million-volt pressurized generators, of the typeshown in the patent to John G. Trump, No. 2,252,668 dated August 12,1941, it is common to use a set of needles, extending in a straight lineacross the width of the belt, as the spray-on corona points. Theseneedles are spaced A -inch from the belt and -inch from each other, anda potential difference, derived from a voltage source, is appliedbetween the needles and the pulley over which the belt passes. Such acharge-spray-on unit, comprising a set of needles and an externalvoltage source, may be provided either at the grounded pulley or at thepulley within the highvoltage terminal, or at both pulleys. Usually achargespray-on unit is provided at the grounded pulley only.

In order that electric charge may be transferred between ground and thehigh-voltage terminal, a chargecollector unit must be provided for eachcharge-spray-on unit. Thus, if a charge-spray-on unit is provided at thegrounded pulley, a charge-collector unit must be provided within thehigh-voltage unit. The charge-collector unit differs from thecharge-spray-on unit in that no external voltage source utilized in thecollection of charge from the belt. The voltage difference between thebelt and the collector corona points is created solely by the charge onthe belt. This voltage dilference is thus limited by the charge densityon the belt, and so the collector points must be placed sufiicientlyclose to the belt in order to initiate corona therebetween. Theproximity required aseegesa is such as to render impractical the use ofneedles of the type usedin the spray-on unit.

Owing to the thickness of the needles, the needle points must be spacedapart at leastJ/ inch,.or thereabouts. Corona is initiated at eachpoint, and each such corona discharge fansv out in a conefrom the pointtowards the belt. In the spray-on unit, the needles are spaced from thebelt sufliciently so that the cross-sectionofeach cone at thebeltsurface islarge enough to-overlap the cross? section ofiadjacent' cones,withthe resultthat charge is sprayed uniformly acrossthe. width. of thebelt. However,,when the needlesare very close to the belt, as. they.

must be in the. collector. unit, the cross=sections atthe belt surface.of. adjacent corona-discharge cones are spaced apart, and. those.portions'of. the-charge on.the beltwhichz.

lieinthe. intervening; gaps may not-bescollected. by the collector.needles...

Moreover, the proximity requiredbetween the collector:

needles. and the. belt tends I. to cause.: gouging of. the. belt:- bytheneedles. Atthespray-on. unit, the. belt hugs thepulley as it passesbytheneedles, so that the beltdoes not strike thev needles- But atlthecollectorv unit, the belt is remote from the pulleyandtendsto vibrateagainst the needles. Furthermore, the spray-on needlesmay al-. ways bespaced from the belt sufiiciently toavoid anydanger of gouging, sinceany increase inneedle-to-belt spacing may be compensated for byincreasing the voltage supplied, by the external voltage source, betweenthe pulley and the needles. spacing is not available at the collectorunit, as herein-' before noted.

For the foregoing reasons, it has been customary to use.

a wire screen as the-.collectorpoints. A SOTmesh screen, for example,provides a row of points which are very close together, and the screenis sufliciently pliant to avoid gouging the belt. However, .wire screensof this nature burn out in .a relatively short time, and mustbe replacedfrequently. Since the wire screen is located in the highvoltage terminalof the generator, the replacement thereof is an elaborate andtime-consumingprocess.

My invention. overcomes all the-foregoing difliculties by providing acollector unit in which a set of corona points placedsufficiently closeto the belt to initiate a corona discharge is used in combination with acorona probe positioned within the ionized region of the coronadischarge but spaced sufficiently from the'belt to avoid contacttherewith. The corona probe is positioned in advance oftheset of coronapoints with respect to the. approaching belt,.,so that in general themajor portion of'thecun rent removed. from the belt will becarriedby thecorona probe. The set. of coronav points, which. initiates. thecoronadischarge, should be flexible, since it will probably come in.contact with the belt at least occasionally, and inmost cases it willactually ride against the belt surface with slight pressure. The pointsshould be sharp and closely spaced, since they must initiate the coronadischarge. Hence a wire screen is generally used as this set of coronapoints. The corona probe, which carries the major portion ofthe currentcollected, should be fixed in position, so that it always lies inthesame part of the ionized region of the corona-discharge. However, sincethecoronaprobe does not need to be capable of initiating the coronadischarge, the sharpness and spacing of the points of the corona probeare not especially critical.

Hence, the corona probe may comprise a row of needles or even aknife-edge. Moreover, .even if the sharpness of. the needle-pointsorknife-edge is adversely affected. by heavy currents collected bythecorona. probe, ,no harm is done, since the coronaprobe does notinitiate. the. coronadischarge. The wire screen, whichdoes initiate thecorona discharge, is protected againstdamage, since it collects onlyrelatively light currents.

My invention may best be understood. from the following detaileddescription with reference to the accompany.- ingdrawings, in which: i

This flexibility in needle-to-belt a Fig 1 is a diagram representingschematically the essential'components of an electrostatic generator;

Fig. 2 is a detail in vertical section of a charge-collector unitconstructed in accordance with my invention, and including thecombination of a set of needles with a wire screen;

Fig. 3 is a plan view of the set of needles of the chargecollectorunitof Fig. 2';

Fig. 4 is a plan view of the assembly of the set of needles, the. wirevscreen, and the supporting, bar in the charge-collector unit of Fig. 2,the supporting bar being partly broken away to show the wire'screen moreclearly;

Fig. 5 is a graph illustrating the current collected by the needles andthecurrent collected by the wire screen as a function of they totalcurrent available for collection; and

Fig. 6 is a diagram, similar to that of Fig. 1, representingschematically the essential components of an electro staticgeneratorinwhich. the down-run of'thebelt is used to. convey electriccharge to ground.

Referring more particularly to the drawings, and, first to Fig.lthereof, therein are shown diagrammatically such parts of anelectrostatic generator as are necessary to an understanding of thisinvention. The said generator is of the type shown in the patent to JohnG. Trump, No. 2,252,668, dated August 12, 1941. The invention hereinclaimed may be applied to the said type of electrostatic apparatus,.butmy invention is not necessarily limited to use with suchtype'of'apparatus.

lnFigl the electrostatic generator, merely diagrammatically shown,comprises the main high-potential electrode-ll consisting of a hollowshell of conducting niateriaLysuch as stainless steel, this being of agenerally rounded and approximately hemispherical shape and free fromexternalprojections. The electrode rests on a ringor terminal plane 2 ofconductive material, outer exposed surface of which is rounded, the saidterminal plane, in turn, being mounted on the top of any suitablenumber, such as three, of spaced elongated pillars or columns, notshown, which may be similar to those in the said patent to Trump, saidpillars or columns being of insulating material of high dielectricstrength.

Thebases of these columns rest on the base plate 3 of conductivematerial which is supported by suitable brack ets attached to the innerwalls of a tank 4 which provides a chamber completely enclosing all theparts of the gen erating apparatus, but leaving a substantial clearancebetween its walls and the electrode and insulating columns. The saidtank is desirably filled with a gas of many atmospheres pressure which,for air, may be about 400 pounds per square inch.

The'charge-carrier may be of. any suitable type, but isherein shown asin the form of an endless belt 5 of such construction that the chargeson its surface are longitudinally insulated from each other, the belt 5herein being of insulating material, such as a multipleply; rubberfabric. At its lower end the belt 5 passes over a metallic drivingpulley 6 journaled in suitable brackets on the base plate 3. The saidpulley 6 is driven by a suitable motor, not herein illustrated, to whichcurrent is supplied by conductors entering the walls of the tank 4through a suitable bushing.

The belt 5 runs vertically upward and then downward in a parallel line,passing into and out of the hollow electrode 1 and over a metallicpulley 7 within the electrode shell, the said pulley 7 being journaledin suitable brackets supported by the terminal plane 2..in amanner notnecessary to illustrate or to refer to further.

Atthe lower end of the belt 5 charges of one sign are established on themoving belt 5, and at its upper end the charges carried thereby areremoved and transferred to the electrode 1. The charging of thedownrunning side of the belt 5 with charges of theopp'osite assumessign, for the purposes of further increasing the total charge deliveredto the electrode 1 is optional and is not necessary to the function ofmy invention.

The charge spraying corona points are indicated diagrammatically at 8and the charge removing apparatus is indicated diagrammatically at 9.The charge spraying corona points 8 may be of the conventional type, andno further reference to them is necessary to an understanding of myinvention.

The charge-collecting apparatus 9 of my invention is shown in detail inFigs. 2, 3 and 4. A wire screen 10 and a row of needles 11 are bolted toan electrostatic shield 12 and the electrostatic shield 12 is bolted tothe terminal plane 2, so that the entire charge-collecting assembly 9,including the screen 10, the needles 11, and the shield 12, areelectrically connected directly to the high-voltage terminal 1. The rowof needles 11 is shown in detail in Fig. 3, and may comprise a row ofstainless steel needles each of which is about A -inch in diameter atthe base. The needles may be soldered to one another near theirrespective bases so as to form a rigid comb-like structure extending thewidth of the belt 5, the needle points being spaced about -inch fromeach other. An apertured metal tab 13 may be soldered to each extremityof the row of needles 11, by means of which tab 13 the row of needles 11is supported.

The wire screen 10, which may comprise a SO-mesh stainless steel screen,is placed upon the row of needles 11 as shown in Fig. 4. The wire screen10' should extend at least the width of the belt 5, and preferablyextends beyond the extremities of the row of needles 11 and up to theapertures 14 in the tabs 13. The wire screen 10 is held in place bybeing clamped between the tabs 13 and a supporting bar 15, which isplaced over the wire screen 10 so that the apertures 16, in thesupporting bar 15 are in alignment with the apertures 14 in the tabs 13,as shown in Fig. 4.

Bolts 17 are inserted through the apertures 16 in the supporting bar 15,through the apertures 14 in the tabs 13, and are screwed intocorresponding tapped apertures 18 in the electrostatic shield 12, sothat the entire unit 9 is thus clamped together. The entire unit 9 is inturn supported on the terminal plane 2 by bolts 19.

The wire screen 10 should be so positioned as to ride against the belt 5'with slight pressure, as shown in Fig. 2. The needles 11 should be sopositioned that their points are spaced from the belt 5, a distance of,5 inch being a suitable distance when negative charge is to becollected from the belt 5 at about 350 p.s.i. pressure. The appropriatespacing between the belt 5 and the needle points 11 may vary with eachparticular generator and with pressure and composition of the gas, andsuch spacing may be greater than -inch in some cases, particularly whenpositive charge is to be collected from the belt 5.

Referring again to Fig. 2, the function of the wire screen 10 is toinitiate ionization in the gas intervening between the collector unit 9and the belt 5. Since the edge of the wire screen 10 rides against thebelt 5 in vertical contact therewith, the potential difference producedbetween the wire screen 10 and the belt 5 by the charge on the belt 5 isadequate to initiate this ionization or corona discharge. It should benoted that despite the fact that the edge of the wire screen 10 appearsto be in contact with the belt 5, there will actually always be a thinlayer of gas therebetween.

Once the corona discharge has been initiated between the 'wire screen 10and the belt 5, the region of ionization extends out from the wirescreen 10 towards the approaching belt 5 through a relatively largevolume the boundary of which is indicated approximately by the brokenline in Fig. 2. The needles 11 are so positioned that the needle pointsextend into this region of ionization, and hence the needles are able tocollect electric charge from the belt 5. Furthermore, once the passageof electric charge through the ionized region between the belt 5 and theneedles 11 has been initiated in the foregoing manner, the needles 11may help to sustain the discharge, even though they would not have beenable to initiate the discharge in the first instance.

Fig. 5 is a graph showing the current collected by the needles and bythe wire screen in a 2-million-volt negative-potential pressurizedgenerator, the atmosphere surrounding the generator being percentnitrogen and 20 percent carbon dioxide at 350 psi. gage. The currentscollected are shown as a function of the current available forcollection, the latter quantity corresponding roughly to the net chargeon the belt. The graph of Fig. 5 shows that as the current delivered bythe belt is increased, the needles carry a greater proportion of thecurrent collected; and at the higher currents the needles carry most ofthe current, while the current collected by the wire screen neverexceeds about 25 microamperes.

Owing to the fact that a wire screen, such as that shown at 10 in Fig.2, is quite flexible, it is diflicult to maintain the edge, which ridesupon the surface of the belt, in a straight line. As a result, thecurrent collected by the screen may be non-uniform along its edge, andsometimes portions of the edge of the screen do not collect any currentfrom the belt. This phenomenon markedly reduced the total currentcollected from the belt, where such a screen provides the onlychargecollecting corona points. However, in accordance with my inventionand as shown in Fig. 2, the ionized region produced by the wire screen10 will generally extend across the entire length of the row of needles11 even if some portions of the edge of the screen 10 are collecting nocurrent. Since the needles 11 carry the major portion of the currentcollected from the belt 5, the total current collected is notappreciably altected by any non-uniformity in the current collected atthe screen 10.

The invention also tends to stabilize the current collected, since anyfluctuation in the current collected by the needles 11 is compensatedfor, at least in part, by an opposing fluctuation in the currentcollected by the screen 10. Thus, if the current collected by theneedles should drop momentarily, there will be a corresponding increasein the charge density remaining on the belt when it passes the edge ofthe screen, so that the current collected by the screen will risemomentarily. Similarly, a transient rise in the current collected by theneedles results in a transient drop in the current collected by thescreen.

As hereinbefore stated, in addition to spraying electric charge onto thebelt at ground and collecting the charge within the high-voltageelectrode, electric charge of the opposite sign may be sprayed onto thebelt within the high-voltage electrode and collected at ground. Such anarrangement is shown in Fig. 6, wherein the voltage source required tospray charge onto the belt 5 within the high-voltage electrode 1 isderived by causing the current collected by the collector points 9 topass through a resistor 21 which electrically connects the collectorpoints 9 to the high-voltage electrode 1. Charge-sprayon points 22 areelectrically connected directly to the high-voltage electrode 1, and theupper pulley 7, which is opposite said spray-on points 22, iselectrically connected directly to the collector points 9, so that thepotential difference across the resistor 21, resulting from the currenttherethrough, also appears between the spray on points 22 and the upperpulley 7'. The polarities are such that the charge sprayed by thespray-on points 22 is of a polarity opposite to that of the chargecollected by the collector points 9.

The electric charge carried by the down-running side of the belt 5 isremoved at ground by collector points 23 m the same manner as theelectric charge carried by the up -runn1ng side of the belt is removedby the collector points 9 within the high-voltage electrode 1.Accordlngly, the grounded collector points 23 shown in Fig.

6 may be constructed in accordance with my invention: and-as showninFigs. 2, 3, and 4.

Similarly, the spray-on points, Sshown in Figs. 1 and 6,- and thespray-on points-22 shown in Fig. 6,.may be. constructed inaccordance-with rny invention. Such a construction might be desirableif, for example, the: external power' supply used to createthe coronadischarge. from the spray-on-noints to the belt-S had only a'limitedlvoltage output. However, at the present time the. major application ofmy invention isinthe collection ofcharge from the belt, where thevoltage difierence across the corona gap is necessarily limited.

Having thus described the. principlesof the invention, together with anillustrative embodiment, thereof,. it is: to be" understood thatalthough specific. terms are: employed, they are used in a generic anddescriptive sense, and not for purposes of limitation, the scope of theinvention being set forth in the following claims;

I claim:

1 Apparatus for transferring electric chargebetween a traveling chargecarrier and an electrode, between which a voltage difierence exists,comprising in combination a set ofcorona points electrically connectedto said electrode and supported sufiiciently close to said traveling.charge carrier so as to. initiate a corona discharge between said set ofcorona points and said traveling charge carrier, and a corona probeelectrically connected to said set of corona points and supportedwithinthe. ionized region of said' corona discharge but sutficientlyremote from said-traveling charge carrier so as to avoid. any contacttherewith, said corona probe being so positioned that said chargecarrier in its traveling movement passes by said corona probe beforepassing by said: set of corona points.

2. Apparatus in accordance with claim 1, wherein said corona probecomprises a second set of corona points.

3. Apparatus in accordance with claim 1,. wherein saidv corona probecomprises a knife-edge.

4. Apparatus in accordance with claim 1-, wherein said voltagedifiercnce is produced by an external power supply and wherein theapparatus produces a net electric charge on the charge carrier.

5. Apparatus in accordance with claim 1, wherein said voltage differenceis produced by a net electric charge on the charge carrier and whereinthe apparatus collects electric charge from the charge carrier.

6. Apparatus for collecting electric charge from the traveling belt ofan electrostatic generator, comprising in combination a set of coronapoints supported sufliciently close. to said belt so as to initiate acorona discharge between said set of corona points and said belt, and acorona probe electrically connected to said set of corona points andsupported within the ionized region of said corona discharge butsufiicien-tly remote from said belt so as to avoid contact therewith,said corona probe being positioned in advance of said set of coronapoints with respect to the approaching belt.

7. Apparatus for collecting electric charge from the traveling.insulating belt of an electrostatic generator, comprising in.combination. a; flexible sfitzof; closely spaced, coronaepoints,supported. sufficiently. close, to, said; belt so and being positionedin advance of; said set of corona points with respect to. theapproaching belt.

Apparatus for collecting electric charge from the traveling insulatingbelt of an electrostatic generator, com prising in. combinationaflexible setof corona points supported almost incontact with the;charge-carrying surface,

ofthe belt, whereby anrionized region is -p roduced; intheneighborhoodcf said corona oints: and ext nd g oward the approaching.belt; and a fixed set, of; corona. points electrically connectedto saidflexible setof; corona points and positioned 'so asto. probe, saidionized region but sufii cientlyremotefromsaid; heltso; to avoid contactthere; with, whereby said fixed set of coronav points collects electriccharge from. said belt prior tov thev arrival of said belt at saidFlexible set of corona points.

9. Apparatus for. collecting, electric charge from the travelinginsulating belt of an electrostatic generator, corn prising incombination a flexible setof corona points riding with slight pressureagainstthe charge-carrying surface of the belt, whereby an ionizedregion is produced in the neighborhood of said corona pointsandextending towards, the approaching belt; and a fixed set of coronapoints electrically connected to said flexible setof corona Points. andpositioned so. as to probe said, ionized region but suf ficiently remotefrom said belt so as to avoid contact therewith, whereby said fixed setof corona points collects electric charge from said belt prior to the.arrival of said belt at said flexibleset of corona points.

10. Apparatus for collecting electric charge from the travelinginsulating belt ofan electrostatic generator, comprising in combinationa flexible screen of; conductive material one edge of which rides withslight pressure against the charge-carrying surface of the belt, wherebyan ionized region is produced in the neighborhood of said edge andextending towards the approaching belt; and a fixed set of conductiveneedles the points whereof are Within said ionized region but spacedfrom said belt so as to avoid contact therewith, the points of said setof needles being positioned in advance of said edge of said screen withrespect to the approaching belt, said needles being electricallyconnected to said Screen.

References Cited in the file of this patent UNITED STATES. PATENTS836,576 Hardwicke Nov. 20, 1906. 1,275,585 Morgan Aug. 13, 19182,333,213 Slayter Nov. 2, 1943 2,449,972 Beach Sept, 28, 1 948 2,503,224Trump Apr. 4, 19 50

